Linus Torvalds [off-list ref] wrote:

If we want to have any kind of confidence that the hash is reall
yunbreakable, we should make it not just longer than 160 bits, we should
make sure that it's two or more hashes, and that they are based on totally
different principles.

Sorry, it has already been shown that combining two difference hashes
doesn't necessarily provide the security that you would hope.

I think what hasn't been discussed here is the cost of actually doing
the comparisons.  In other words, what is the minimum number of
comparisons we can get away and still deal with hash collisions
successfully?

Once we know what the cost is then we can decide whether it's worthwhile
considering the odds involved.
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